Patient populations are studied to clarify the long-term effects of exposure to medical irradiation, cytotoxic agents and other therapeutic drugs. Multiple primary cancers are studied to evaluate late effects of therapy and to generate hypotheses about common etiologies. Patients genetically predisposed to cancer are studied for insights into mechanisms of carcinogenesis. Principal findings were a (1) Solid cancer risk increased over time following bone marrow transplantation, and was higher for patients treated as children; (2) an excess of bone and soft tissue sarcoma among bilateral retinoblastoma patients is ascribable to high-dose radiation; (3) total cancer mortality was not elevated 5 years or more following treatment for hyperthyroidism; however, suggestive associations were seen between thyroid cancer mortality and I-131 treatment; (4) salivary gland tumors were linked to childhood radiotherapy for enlarged tonsils; (5) the p53 mutational spectrum associated with cyclophosphamide-related bladder cancer differed from patterns reported for sporadic, smoking-related, and Schistosomiasis linked tumors; (6) men with testicular cancer are at increased risk of second malignant neoplasms for over two decades following initial diagnosis; (7) no significant excess incidence of cancer was observed among infertility patients treated with radiation, but there was some suggestion of an increased risk of colon cancer; (8) the incidence of Hodgkin's disease was elevated following allogeneic bone marrow transplantation.